Making jam – the Science

In our garden we have several fruit trees, including two fig trees. Unfortunately, I’m not a big fan of fresh figs and they tend to spoil before I eat them. Luckily, I do like fig jam! So I’ve tried to convert all my figs into jam. What’s more, making jam is pretty easy and you need very little to make it work.

fig tree
Our fig tree in winter.

What is a jam?

Let’s start with the verb ‘jamming’, it means “squeeze or pack tightly into a specified space”. That’s actually what a fruit jam is as well, a lot of fruit packed tightly into a jar!

When making jam the fruits are boiled down and mixed with sugar, giving a sweet tasting product. Jams were made to preserve fruits back in the days when refrigeration was not yet available. The high sugar content of jams prevents micro organisms from growing in the jam! As an example, my fresh figs started moulding in less than a week of storage in my fridge, whereas my fig jam has been standing strong for more than half a year now!

How does making jam work?

Making jam always starts of by boiling your fruits gently. By cooking them you will break down the structure of the fruit. The cell walls will break down and pectin, which sits in these walls, will be freed. Pectin is a large molecules. These molecules can prevent molecules around it from moving freely. This will make a gel, which is very convenient when making jam.

However, there’s not enough pectin in fruit to make pure boiled fruit set like a gel. That is why sugar is added as well. Sugar will dissolve in water and that way reduce the number of freely moving water molecules. Think of a sugar syrup, it doesn’t flow as freely anymore as pure water does or water with little sugar. This combination of sugar and pectin will make your jam firm.

When reading about making jam I discovered that there is another cool way to improve your jam. Pectins gel best at a specific pH range, they prefer a slightly acidic evironment. By adding some lemon juice you can actually improve the effectiveness of the pectin. I haven’t been able to test this yet, but I’d be curious to hear results!

making jam: fig and grape jam in bowls
Grape jam to the left & fig jam to the right

Making jam, three recipes

I’ve made jam with three types of fruit so far: grape, fig and pineapple. My personal favorites are the last two, fig and pineapple. All three of them are made in a similar way:

Making jam - the Science
  • 1 kg of fruits (fresh pineapple: cut into pieces and remove the skin or grapes or figs: remove the skin from the figs, leave in the seeds etc.)
  • 500g of sugar
  1. Boil the fruits down on a low heat. It's best not to add any water at this point. However, if you need to because your fruits are caramelizing (getting brown), add a very thin layer of water.
  2. Add the sugar once everything has boiled down and keep on boiling on a low heat. Do not put it on a high heat, it will start browning and sticking to the bottom of the pan.
  3. Your jam is ready when it stays firm once it's cooled down. Any easy way to test is to take a small bit out of the pan and leave it to cool.

I like my jams with all seeds and pieces of fruit still in there. However, I must admit that when I made this with grapes with seeds in them, it didn’t really improve the jam. It’s better to remove the seeds, but that might make it harder for you to set your jam since you’re also removing skins and possibly pectins. Best would be to use grapes without seeds, that should give you no trouble at all! The seeds in the figs are not a problem in the jam, nor are those of the pineapple.


That’s it for the science basics of making jam. Hope you learned something and please let me know if you have further questions, I’d love to answer them!

homemade grape jam
My grape jam, the taste is perfect, but if you look carefully you can see the seeds, those are a little too crunchy for a nice jam.

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